This invention relates to a curable siloxane-epoxy molding composition with improved crack resistance. In another aspect the invention relates to a method for improving the crack resistance of siloxane-epoxy moldings.
Siloxane-epoxy copolymers and blends are well known for coating, casting, and molding applications. Siloxane-epoxy copolymers formed by the condensation of silanol and hydroxyl groups present in the epoxy resins are described in such patents as U.S. Pat. No. 3,170,962 for use in coatings and castings. The cure of such copolymers is obtained from the reaction of residual epoxy functionality. Siloxane-epoxy blends are known from U.S. Pat. No. 3,368,893 for use as binder resin in zinc oxide containing photoconductive compositions employed in an electrophotographic method of preparing etchable printing plates. The blends were cured by contact with solutions containing aluminum catalyst. Generally, improved properties such as antichalking in coatings, humidity resistance in electrical insulation, and abrasion resistance in binder applications are recognized in the materials.
Both types of siloxane-epoxy materials are also known in molding compositions. In U.S. Pat. No. 3,842,141, siloxane-epoxy copolymers are prepared by heating epoxide resins which contain hydroxyl groups with a silanol containing phenylsiloxane resin in a solvent to effect a silanol-hydroxyl condensation. Water formed by condensation and solvent are removed and the solid copolymer is ground before mixing with other ingredients such as curing agents, release agents, pigments, fillers, and catalysts to obtain a molding compound. Epoxy resins containing hydroxyl functionality such as the polydiglycidyl ether of bisphenol A are employed in the production of copolymers, but such resins as epoxidized novolacs are taught to be inoperable. The cure of such copolymers in molding compounds is obtained from the reaction of residual epoxy functionality with added curing agents such as anhydrides.
Molding compositions based on siloxane-epoxy blends are known from U.S. Pat. No. 3,971,747. In this case a curable molding composition is prepared by mixing an epoxy resin, a silanol containing phenylsiloxane resin, an aluminum catalyst and other ingredients such as release agents, pigments, and fillers. The aluminum catalyst facilitates reaction between the epoxy group and the silanol to effect a cure when the blend is molded and heated. Epoxy resins utilized in blends are not limited to those containing hydroxyl functionality and epoxidized novolacs are employed in the siloxane-epoxy blended molding compounds.
The siloxane-epoxy blended molding compound combines many of the features of siloxane and epoxy resins such as moisture resistance, salt atmosphere resistance, high strength, and strong lead seal. It has been used commercially to encapsulate electronic devices. However, one of the problems with siloxane-epoxy blended molding compounds is the fact that their normal shock crank resistance is not sufficient for some applications. In these applications the electrical device must function in an environment of rapid temperature fluctuations. When the encapsulant on a device cracks, failure of the device often results. Consequently, it is important that the crack resistance of siloxane-epoxy blended molding compounds be improved.
Applicants have discovered that when an automatic polyepoxide resin (D) which has more than one epoxide per molecule, has a Durran's melting point between 60.degree. and 110.degree. C., and has at least 38 weight percent based on the weight of (D) of a polydiglycidyl ether of a bisphenol is employed in a siloxane-epoxy blended molding compound containing silica fillers, improved crack resistance is surprisingly obtained. It is known from the Handbook of Epoxy Resins* (Henry, L. and Neville, K., N.Y., McGraw-Hill, Inc., 1967, p. 16-3) that higher molecular weight polydiglycidyl ether of bisphenol A may, under conditions of reaction not leading to reaction through groups other than the epoxy (i.e., hydroxyls), be employed to impart some degree of flexibility to cured systems. Since it is expected that reaction occurs between silanols and hydroxyl groups as well as between silanols and epoxy groups in siloxane-epoxy blends employing polydiglycidyl ether of bisphenol A, it is unexpected that a more flexible or crack resistant cured molding could be obtained. FNT *Which is hereby incorporated by reference.
It is known from U.S. patent application "Method of Improving the Crack Resistance of Siloxane Molding Compositions" by Robert C. Antonen, Ser. No. 773,364 filed Mar. 1, 1977, and now U.S. Pat. No. 4,125,510, and assigned to the assignee of the present invention, that small amounts of epoxy resins (4 to 15 percent by weight based on the combined weight of silicone and epoxy resin employed) improve the crack resistance of silicone molding compounds. These silicone molding compounds modified with a small amount of epoxy resin are not within the claims of the present invention which require at least 40 weight percent epoxy resin (based on the combined weight of silicone and epoxy resin employed) and which require an aluminum catalyst. Moreover, this reference does not teach or suggest the specific epoxy resin compositions required in the present invention to improve the crack resistance of siloxane-epoxy molding compounds.